A living polymerization process is one that can produce polymers of predetermined molecular weight and a narrow molecular weight distribution containing one or more monomer sequences, the length and composition of which are controlled by stoichiometry of the polymerization reaction and degree of conversion. Accordingly, homopolymers, random co-polymers and block co-polymers, as well as polymers of more complex architectures, including branched homopolymers and co-polymers of low polydispersity may be produced with a high degree of control. Characteristics of a living polymerization process include:                “1. Polymerization proceeds until all of the monomer has been consumed. Further addition of monomer results in continued polymerization.        2. The number average molecular weight (or the number average degree of polymerization) is a linear function of conversion.        3. The number of polymer molecules (and active centers) is a constant which is sensibly independent of conversion.        4. The molecular weight can be controlled by the stoichiometry of the reaction.        5. Narrow molecular weight distribution polymers are produced.        6. Block copolymers can be prepared by sequential monomer addition.        7. Chain end-functionalized polymers can be prepared in quantitative yield.”(Quirk and Lee (Polymer International 27, 359 (1992))        
Commercial processes for the production of living polymers typically employ anionic initiators. In contrast, free radical polymerization processes possessing living characteristics have only recently been developed. (See, for example, published International Application WO 98/01478.). Such free radical processes use addition-fragmentation chain transfer agents to facilitate reversible chain transfer so that the polymerization has living characteristics. This type of polymerization has been termed reversible addition-fragmentation-termination (RAFT) polymerization.
WO 98/01468 discloses a class of dithiocarboxylic esters for use as chain transfer agents in free radical living polymerization processes. These chain transfer agents have the structure of formula C or D:
wherein
Z is selected from the group consisting of hydrogen, chlorine, optionally substituted alkyl, optionally substituted aryl, optionally substituted heterocyclyl, optionally substituted alkylthio, optionally substituted alkoxycarbonyl, optionally substituted aryloxycarbonyl (—COOR″), carboxy (—COOH), optionally substituted acyloxy (—O2CR″), optionally substituted carbamoyl (—CONR″2), cyano (—CN), dialkyl- or diaryl-phosphonato (—P(═O)OR″2), dialkyl- or diaryl-phosphinato (—P(═O)R″2), and a polymer chain formed by any mechanism;
Z′ is a m-valent moiety derived from a member of the group consisting of optionally substituted alkyl, optionally substituted aryl and a polymer chain;
where the connecting moieties are selected from the group that consists of aliphatic carbon, aromatic carbon, and sulfur;
R is selected from the group consisting of optionally substituted alkyl, an optionally substituted saturated, unsaturated or aromatic carbocyclic or heterocyclic ring; optionally substituted alkylthio; optionally substituted alkoxy; optionally substituted dialkylamino; an organometallic species; and a polymer chain prepared by any polymerization mechanism; in compounds C and D, R. is a free-radical leaving group that initiates free radical polymerization;
R” is selected from the group consisting of optionally substituted C1-C18 alkyl, C2-C18 alkenyl, aryl, heterocyclyl, aralkyl, alkaryl wherein the substituents are independently selected from the group that consists of epoxy, hydroxy, alkoxy, acyl, acyloxy, carboxy (and salts), sulfonic acid (and salts), alkoxy- or aryloxycarbonyl, isocyanato, cyano, silyl, halo, and dialkylamino;
p is 1 or an integer greater than 1; when p is greater than or equal to 2, then R═R′; and m is an integer greater than or equal to 2.
Although these compounds function effectively as chain transfer agents, they share one major disadvantage with the entire class of sulfur-based compounds, a characteristic disagreeable odor. While this is an inconvenience during synthesis and/or production of the polymer in the laboratory, toxicity of the compounds and/or difficulty in handling large quantities of compounds having such a powerful and offensive odor may preclude use in industrial scale processes. In addition, residual odor in the product may be sufficiently objectionable to bar use in its intended application, or at least limit the amount that can be tolerated in a polymer for commercial use. It can be seen, then, that there is a continuing need for chain transfer agents lacking the characteristically offensive sulfur odor.